1. Field of the Invention
The present invention relates to an illumination device having a light source and a light guide element for guiding a light flux emitted from the light source, and having particular characteristics in an exit surface of the light guide element, and to an image reading apparatus using the illumination device.
2. Description of Related Art
Known arrangements developed for the illumination device used for illuminating a read original in image reading apparatuses for information processing apparatuses such as facsimiles, photocopiers, and so forth include discharge tubes such as fluorescent lamps or LED arrays which are made up of an array of a great number of LED chips. Particularly, in recent years, increased use of the facsimile in the home has necessitated even more compact and inexpensive products, and thus there are a great many products which use LED arrays. Also, there are arrangements wherein LED chips are provided at the ends of a rod-shaped light-transmissive member made of transparent glass, resin, or the like, so that the light flux emitted from the LED chip is linearly extended by making use of the internal reflection of the light-transmissive member, and thus cast upon the original document to be read, in order to reduce the number of chips used and to realize even further reductions in cost.
FIG. 28 shows a cross-sectional diagram of such an image reading apparatus. In FIG. 28, reference character A denotes an original document which has an image to be read on the surface thereof. Reference character B denotes a light source for casting light upon the original document A. Reference character C denotes a rod lens array for imaging the light flux reflected from the surface of the original document A onto the surface of a sensor D. Reference character E denotes a housing for holding the light source B and the rod lens array C. Reference character F denotes a glass plate which holds the original document A, and reference character G denotes a substrate upon which the sensor D is mounted.
The light source B is composed of a light-transmissive member formed of a light-transmissive material such as that denoted by 3 in FIG. 28, and LED lamps (not shown) provided on either end thereof.
FIGS. 29(a) and 29(b) show schematic diagrams for explaining the function of the light source B shown in FIG. 28. FIG. 29(a) is a schematic diagram showing a cutaway view of the light source B from the direction indicated by the arrow in FIG. 28, wherein the portion of the original document A upon which light is cast serves as a plane to be illuminated.
In FIGS. 29(a) and 29(b), reference numeral 1 denotes LED lamps fabricated by mounting LED chips in a package formed of resin, ceramics, or the like. Reference numeral 3 denotes a light-transmissive member made of a light-transmissive material such as acrylic resin, for example, reference numeral 4 denotes an entrance surface by which the light flux generated by the LED lamp 1 enters the light-transmissive member 3, and reference numeral 5 denotes an area for reflecting and/or scattering the light flux propagated through the light-transmissive member 3 to project the light flux out of the light-transmissive member 3.
The light flux emitted from the LED lamp 1 and entering the light-transmissive member 3 from the entrance surface 4 of the light-transmissive member 3 is propagated throughout the light-transmissive member 3 by repeatedly reflecting at the inner surface thereof. In the course of such reflecting, the light flux is cast into the above area 5, wherein the light flux is diffused, and the light is cast out of an exit surface thereof facing the above area 5, thus illuminating the reading area of the original document in a linear manner. A light condensing part 7 is formed on the exit surface, thus illuminating the reading area of the original document with a degree of illuminance greater than an arrangement not provided with the light condensing part 7.
In the event that an LED lamp having only a single color light is used for the LED lamp 1, an apparatus for reading monochrome images can be formed, but in the event that reading of color images is to be performed, LED lamps having differing light colors such as red, blue, and green, for example, are provided to the same end together, or an LED lamp which is provided with LED chips of differing light colors within a single LED lamp, or an arrangement wherein LED chips of differing light colors are bonded to a substrate, is used. With these arrangements, the LED chips of the various colors are switched at high speed and caused to emit light, and storing and reading of signal charges in and from the sensor D are performed in synchronism with the above switching.
For example, of the LED chips 1 provided to the entrance surface 4 of the light-transmissive member 3, in the case of causing the red light emitting chip to emit light, the reading portion of the original document is illuminated with a red color. The light which is reflected off of the original document A at this time and is imaged by means of the focusing rod lens array C causes a charge to be stored in the sensor D, this signal serving as red information of the image on the original document. Illumination by LED chips for emitting green light and blue light generates blue information and green information, respectively.
In order to improve capabilities of such an image reading apparatus by raising the reading speed of the image or raising the S/N ratio during reading of the image, the illuminance of the portion of the original document being read must be raised as high as possible. Accordingly, the light flux exiting from the exit surface of the light-transmissive member is preferably emitted so as to be condensed on the portion of the original document to be read. With known arrangements, a light condensing part has been provided at to the exit surface of the light-transmissive member to yield this effect, but in almost all cases, the light condensing part has been of such a circular cross-sectional form. Also, many such light condensing parts have been formed to emit the light flux in a parallel manner, and the light condensing effects have not been sufficient. As shown in FIG. 30, the light condensing part 7 of the light-transmissive member 3 is round in sectional shape, so that the scattered and reflected light from the area 5 becomes a parallel light flux at the exit surface of the light condensing part 7.
However, in the event that sufficient convergence of light on the portion to be read is attempted with a round cross-sectional condensing part, there have been the following problems, i.e., the radius of curvature must be reduced, but doing so also reduces the size of the light condensing part, resulting in an increase in the amount of light flux leaking instead of being cast into the light condensing part, meaning that the resultant illuminance on the portion of the original document to be read is not increased. As shown in FIG. 31, if the exit surface of the light condensing part 7 is narrow, light flux leaks out in directions other than the direction toward the original document.
Also, the angle distribution of the light flux reflected at the reflecting and/or scattering area and heading toward the light condensing part differs between a portion of the light-transmissive member near the LED lamp serving as the light source and a portion of the light-transmissive member farther away from the LED lamp. This has been problematic in that in the event that the form of the light condensing part is uniform throughout, since the state of convergence on the original document differs between the portion near the LED lamp and the portion farther away from the LED lamp, this tends to result in non-uniform illuminance distribution on the original document.
It is an object of the present invention to solve the above problems. More specifically, it is an object of the present invention to solve the problems by using a light condensing part for the illumination device having a freer form such as that expressed by a general polynomial, rather than a circular cross-section.
It is another object of the present invention to achieve even greater effects by making the light condensing part in such a form that, in the polynomial, such that the sign of a coefficient of the quadratic term and/or a coefficient of the quartic term is opposite to the sign of a coefficient of the cubic term.
It is another object of the present invention to obtain uniform illuminance distribution by changing the form of the light condensing part along the longitudinal direction of the light-transmissive member.
It is yet another object of the present invention to obtain an even higher degree of uniformity in illuminance distribution even in cases where the position of light convergence is shifted to the right or left instead of directly above the reflecting and/or scattering area, or where the plane to be illuminated is illuminated from an oblique direction instead of a vertical direction, by forming the light condensing part so as to be non-symmetrical.
It is a further object of the present invention to improve illuminance on the original document and obtain image signals with good resolution, by means of employing the above illumination device in the image reading apparatus.
Other objects and feature of the present invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.